Reactor

ABSTRACT

A reactor for aerating a fluid with a gas comprises a mixing tank ( 12 ) for the fluid and a centrally located vertical draft tube ( 13 ) submerged in the fluid to divide the mixing tank ( 12 ) into an inner chamber ( 21 ) and an outer chamber ( 23 ). The reactor further comprises a motor driven axial flow impeller ( 14 ) located in the draft tube ( 13 ) for circulating fluid downwardly through the inner chamber ( 21 ) and upwardly through the outer chamber ( 23 ). The reactor further comprises an external circuit for withdrawing a portion of the fluid from the mixing tank ( 12 ), aerating the fluid, and returning the aerated fluid to the mixing tank ( 12 ). The aerator is in the form of a venturi device ( 17 ).

FIELD OF THE INVENTION

[0001] The invention relates to a reactor for a two-phase or three-phasesystem.

[0002] The invention has particular application to the aeration of afluid comprising a slurry of mineral particles with air or any othersuitable oxygen-containing gas, as is required by way of example inaerobic bacterial leaching. However, the invention is not restricted tothis application and extends to the aeration of any gas/liquid,gas/liquid/solid, or gas/liquid/solid/microbial systems.

[0003] The invention has the advantages of aerating a fluid with a gasat low energy usage and with high efficiency in terms of gasutilisation.

[0004] The term “aeration” is understood to mean herein the introductionof a gas or gases into a fluid.

BACKGROUND OF THE INVENTION

[0005] Reactors for aeration of slurries have been in use for many yearsin the mining industry. The two major types of reactors are the Pachuca(or air agitated reactor) and the mechanically agitated reactor.

[0006] The Pachuca reactor was initially favoured due to its simplicityof construction and operation but gradually lost favour as reactor sizeincreased. The loss of favour resulted from the large amounts ofcompressed air required for good mineral suspension. Also, the residencetime of air in a Pachuca reactor is too short for efficient masstransfer and Pachuca reactors are prone to channelling of the air. Airagitation, in general, is inefficient because the bubble size forefficient agitation is too large for efficient mass transfer.

[0007] Mechanical agitation has become more widely used, particularlyfor large reactors, as impeller design has become more efficient and ithas become evident that the extra capital cost was more than compensatedfor by the relatively lower energy required for agitation.

[0008] For efficient mass transfer of air to solution it is necessary toobtain a fine dispersion of bubbles in a well mixed system with thebubbles having a long residence time in the reactor. In practice thishas been obtained by passing the air through a high shear turbineimpeller or by introducing the air through a membrane or porousdiffuser. Both these methods are energy intensive, because the air mustbe introduced at sufficient over-pressures to overcome the liquidpressure at the point of injection and to overcome the pressure dropacross the injection opening, membrane or diffuser. Usually, the pointof injection is at the bottom of the reactor and, in particular, in thecase of aerating large vessels, one of the major costs is the capitaland on-going energy costs to compress the air to the pressure requiredfor injection. If the tanks are deeper than about 10 m it is necessaryto install expensive, high pressure, compressors rather thanair-blowers. Additionally, the use of porous diffusers, or spargers, inreactors for slurries can lead to loss of operating time to unblock thediffusers.

[0009] In addition, mechanically agitated reactors become inefficientwhen large amounts of air are required, because the power required todisperse the air in the reactors becomes very large. Further, in thecase of bacterial reactors, the shear forces present at the blade tipsof high speed impellers can damage the bacteria.

[0010] In addition, particularly for gas/liquid/solid systems where itis important to maintain the solids in suspensions the power required tocirculate the fluid in the aerator becomes a significant cost factor.

SUMMARY OF THE INVENTION

[0011] According to the invention there is provided a reactor forintroducing a gas into a fluid comprising, a mixing tank for the fluid,a partition means for dividing the tank into at least two chambers andfor allowing the fluid to flow between the chambers at a lower regionand an upper region of the tank, a pump means located in one of thechambers for circulating the fluid downwards in one chamber and thenupwards in the other chamber, a means for creating a region of reducedpressure in a portion of the fluid, a means for introducing the gas intothe fluid in the region of reduced pressure to aerate the fluid, and ameans for introducing the aerated fluid into the circulating fluid inthe tank.

[0012] It is preferred that the partition means comprises a draft tubeadapted to be submerged in the fluid in the tank, the draft tube havingan open upper end and an open lower end.

[0013] It is preferred particularly that the tank be cylindrical and thedraft tube be located centrally in the tank to divide the tank into aninner chamber and an outer annular chamber.

[0014] It is preferred that the pump means be located in the draft tube.

[0015] It is preferred that the pump means comprises an axial flow pump.

[0016] It is preferred particularly that the axial flow pump comprisesan impeller located in the draft tube.

[0017] It is preferred that the means for creating the region of reducedpressure in the fluid comprises a tubular member which has a region ofrestricted cross-section for imparting a venturi effect to the fluidpassing through the tubular body whereby the velocity of the fluidincreases and the pressure of the fluid decreases in the region ofrestricted cross-section.

[0018] In one preferred arrangement the region of restrictedcross-section is formed by providing a throat in the tubular member. Inanother preferred arrangement the region of restricted cross-section isformed by inserting a restriction in the tubular member.

[0019] It is preferred that the means for introducing the gas into thefluid comprises a porous membrane, holes, or jets.

[0020] According to the invention there is also provided a method ofintroducing a gas into a fluid comprising, circulating the fluid bymeans of a pump means in a mixing tank having at least two chambers thatare in fluid communication at upper and lower regions of the tank sothat the fluid flows downwards in one chamber and upwards in the otherchamber, creating a region of reduced pressure in a portion of thefluid, introducing the gas into the portion of the fluid in the regionof reduced pressure to aerate the fluid, and introducing the aeratedfluid into the circulating fluid in the tank.

DESCRIPTION OF THE DRAWINGS

[0021] The invention is described further with reference to theaccompanying drawings in which:

[0022]FIG. 1 represents schematically a preferred embodiment of areactor formed in accordance with the invention;

[0023]FIG. 2 is a detailed schematic representation of the basic designof a venturi device for use in the reactor shown in FIG. 1;

[0024]FIG. 3 is a detailed schematic representation of a preferredembodiment of a venturi device for use in the reactor shown in FIG. 1;and

[0025]FIG. 4 is a graph of oxygen uptake and oxygen utilization versusair flow for the reactor shown in FIG. 1 and a conventional air agitatedreactor.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The preferred embodiment of the reactor of the invention isdescribed herein in relation to the aeration of a slurry of a mineraland water with air. However, it is noted that the invention is notrestricted to this application and extends to the aeration of any fluidwith or without suspended solids.

[0027] The reactor 11 shown in FIG. 1 comprises a mixing tank 12containing the slurry, a vertical draft tube 13 submerged in the slurry,and a motor driven axial flow impeller 14 located in the draft tube 13near the top thereof. The tank 12 may be of any suitable size. The drafttube 13 has open upper and lower ends 16, 18 and is located centrally inthe mixing tank 12 to divide the mixing tank 12 into an inner chamber 21and an outer annular chamber 23. In use, the impeller 14 induces flow ofthe slurry downwards in the draft tube 13 and then upwards in the outerannular chamber 23. The flow of the slurry is controlled so that themineral particles are kept in suspension.

[0028] The reactor 11 further comprises an external circuit forwithdrawing a portion of the slurry from the mixing tank 12, aeratingthe slurry, and returning the air-enriched slurry to the mixing tank 12.The external circuit comprises a recycle line 6, a pump 15 for pumpingthe slurry around the external circuit, and a venturi device 17 foraerating the slurry. The external circuit is arranged to withdraw slurryfrom an upper section of the mixing tank 12 and to return theair-enriched slurry to a location in the draft tube 13 above theimpeller 14 to optimise mixing of the air-enriched slurry with thecirculating slurry in the mixing tank 12. The external circuit comprisesat least one re-entry nozzle 19 arranged to direct the air-enrichedslurry down the draft tube 13.

[0029]FIG. 2 illustrates the basic design features of the venturi device17. With reference to the figure, the venturi device 17 comprises atubular body 25 having an inlet end 41, an outlet end 43, and anintermediate throat 3 which defines a region of restricted cross-sectionin which there are holes 2 for introducing air for mixing with theslurry. As the slurry flows through the tubular body 25 in the directionindicated by the arrow A the flow rate increases as the slurry entersthe throat 3 thereby creating a region of reduced pressure accordingBernoulli's equation. As a consequence, in order to introduce air intothe region of reduced pressure it is not necessary that the air be athigh pressure and the air can be introduced at low pressure or bynatural aspiration. As the slurry flows from the throat 3 the slurryenters a region of increased cross-section 5 where the fluid velocitydecreases and the pressure increases.

[0030] The region of increased cross-section 5 is shaped to give maximumenergy recovery as the air-enriched slurry expands as it flows from thethroat 3. Furthermore, the design and operating parameters of theventuri device 17 are selected to form air bubbles of optimal size forefficient oxygen mass transfer from the bubbles to the slurry. As aconsequence, a minimal amount of air is required thereby reducing theoperating costs. The design and operating parameters include slurry flowrate, air pressure, and the means of air injection into the slurry.

[0031]FIG. 3 illustrates a preferred embodiment of the venturi device 17for use with a 3,000 liter capacity mixing tank 12 and a 75 mm diameterrecycle line 6. The throat 3 of the venturi device 17 comprises anentrance cone 45 of 25° and an exit cone 47 of 7°. The diameter of thethroat 3 is 25 mm and the diameter of the inlet and outlet ends 41, 43is 75 mm. The holes 2 are located in the exit core 47 of throat 3 andare arranged in 3 circumferential rows spaced 5 mm apart with each rowcomprising 24×1 mm holes.

[0032] It will be clearly understood that the invention in its generalaspects is not limited to the specific details referred to hereinabove.

[0033] The invention is now illustrated by way of reference to thefollowing example.

[0034] A series of experiments was carried out on a conventional reactorcomprising a 3,000 liter mixing tank stirred by an axial flow impellerand having air injection through a 1 mm drilled hole ring spargermounted beneath the impeller and the preferred embodiment of the reactorof invention shown in FIG. 1 comprising a 3,000 liter mixing tankstirred by an axial flow impeller located in a draft tube and having aventuri device returning aerated slurry to a location above the axialflow impeller.

[0035] The tanks contained 8% w/v slurry of a pyrite/pyrhotite tailswhich was being bacterially leached with Thiobacillus ferrooxidans.

[0036] The aeration performance of each tank was evaluated and theresults are shown in FIG. 4.

[0037]FIG. 4 shows the relationships between:

[0038] (a) oxygen uptake in the slurry and air flow into theconventional reactor and the preferred embodiment of the reactor; and

[0039] (b) oxygen utilisation and air flow into the conventional reactorand the preferred embodiment of the reactor.

[0040] The term “oxygen uptake” refers to the amount of oxygen that wastransferred to the liquor and therefore is a direct measure of theextent of aeration. The term “oxygen utilisation” refers to the amountof oxygen that was transferred to the liquor as a percentage of thetotal amount of oxygen introduced into the reactor and therefore is adirect measure of the efficiency of the aeration.

[0041] With reference to FIG. 4, the term “air sparger” refers to theconventional reactor and the term “venturi aerator” refers to thepreferred embodiment of the reactor.

[0042] The results in FIG. 4 show that the aeration performance of thepreferred embodiment of the reactor was significantly better than thatof the conventional reactor. By way of particular example, with thepreferred embodiment of the reactor it was possible to aerate the slurrywith 150 mg O₂/liter of slurry/hour with an air flow of 60 l/min and anoxygen utilization of 50% whereas with the conventional reactor it wasonly possible to aerate the slurry with 150 mg O₂/liter of slurry/hourwith a considerably higher air flow of 150 l/min and a significantlylower oxygen utilization of 20%.

[0043] The power requirements to aerate each reactor type with a givenvolume of air were monitored and scaled up to values representing theanticipated power requirements for aeration in a 1,000 m³ tank. Theresults are shown in Table 1. TABLE 1 Comparison of Aeration PowerRequirements Aeration Power Tank Configuration (Wh/m³ of air)Conventional Reactor 80 with Air Sparger Reactor of Invention 20 withDraft Tube and Venturi

[0044] The results indicate significant power savings with the preferredembodiment of the reactor compared with the conventional reactor.Specifically, the results show that the energy required per m³ of airdelivered was four-fold lower for the preferred embodiment of thereactor than for the conventional reactor. On the basis of the resultsof the energy utilization to achieve an O₂ uptake of 150 mg O₂/liter ofslurry/hour discussed above, the energy required per m³ of oxygentransferred was nine-fold lower for the preferred embodiment of thereactor than for the conventional reactor.

[0045] The preferred embodiment of the reactor of the invention has thefollowing advantages over conventional reactor:

[0046] (i) The gas is supplied at low pressure or by natural aspiration,thereby eliminating the need for expensive high pressure gas compressorsand reducing the reactor power requirements. Significantly, the agitatoris used only to suspend the mineral particles and to circulate theaerated slurry.

[0047] (ii) The gas is injected or naturally aspirated at a point in theventuri device where the fluid velocity is high. This creates very smallbubbles thus improving mass transfer of oxygen into solution. As aconsequence, the operating costs are reduced because the air requiredfor the reactor is minimised.

[0048] (iii) The aerated slurry is returned to the mixing tank above theimpeller in a central draft tube at low pressure. As a consequence,operating costs are reduced because the pumping power required forcirculating the slurry is minimised.

[0049] (iv) The capital cost of the reactor is minimised since there areless internal parts in the mixing tank. Furthermore, large reactors canbe built leading to economies of scale.

[0050] (v) Maintenance costs and downtime are minimised since there arefew parts to fail inside the mixing tank. Servicing the externalcomponents is simple since a single aeration device can be shut down forservice without affecting the reactor's overall performance. Replacing ablocked aeration device can be done quickly with the minimum ofinterruption to the process.

[0051] (vi) The invention is suited for efficient gas supply and solidssuspension in a gas-liquid-solid system or for efficient gas supply to agas-liquid system. An example of the use of the invention is suspendingand aerating a reacting slurry of mineral particles, as in bacterialleaching. Other uses include the bio-methanation of synthesis gas,aerobic digestion of sewage or other sludges and the production ofsynthetic rutile as in the Becher process. Its use, however, is notlimited to these areas.

[0052] Many modifications may be made to the preferred embodiment of thereactor described herein without departing from the spirit and scope ofthe invention.

[0053] By way of example, whilst the impeller 14 is located near the topof the draft tube 13 in the preferred embodiment, the invention is notrestricted to such an arrangement and the impeller 14 may be located atany suitable location along the length of the draft tube 13.

1. A reactor for introducing a gas into a fluid comprising, a mixingtank for the fluid, a partition means for dividing the tank into atleast two chambers and for allowing the fluid to flow between thechambers at a lower region and an upper region of the tank, a pump meanslocated in one of the chambers for circulating the fluid downwards inone chamber and then upwards in the other chamber, a means for creatinga region of reduced pressure in a portion of the fluid, a means forintroducing the gas into the fluid in the region of reduced pressure toaerate the fluid, and a means for introducing the aerated fluid into thecirculating fluid in the tank.
 2. The reactor defined in claim 1 ,wherein the partition means comprises a draft tube adapted to besubmerged in the fluid in the tank, the draft tube having an open upperend and an open lower end.
 3. The reactor defined in claim 2 , whereinthe tank is cylindrical and the draft tube is located centrally in thetank to divide the tank into an inner chamber and an outer annularchamber.
 4. The reactor defined in any one of the preceding claimswherein the pump means comprises an axial flow pump.
 5. The reactordefined in claim 4 , wherein the axial flow pump comprises an impellerlocated in the draft tube.
 6. The reactor defined in claim 1 , whereinthe means for creating the region of reduced pressure in the fluidcomprises a tubular member which has a region of restrictedcross-section for imparting a venturi effect to the fluid passingthrough the tubular member whereby the velocity of the fluid increasesand the pressure of the fluid decreases in the region of restrictedcross-section.
 7. The reactor defined in claim 6 , wherein the region ofrestricted cross-section is formed by providing a throat in the tubularmember.
 8. The reactor defined in claim 6 , wherein the region ofrestricted cross-section is formed by inserting a restriction in thetubular member.
 9. The reactor defined in claim 1 , wherein the meansfor introducing the gas into the fluid comprises a porous membrane,holes, or jets.
 10. A method of introducing a gas into a fluidcomprising, circulating the fluid by means of a pump means in a mixingtank having at least two chambers that are in fluid communication atupper and lower regions of the tank so that the fluid flows downwards inone chamber and upwards in the other chamber, creating a region ofreduced pressure in a portion of the fluid, introducing the gas into theportion of the fluid in the region of reduced pressure to aerate thefluid, and introducing the aerated fluid into the circulating fluid inthe tank.
 11. A reactor for introducing a gas into a fluid comprising, amixing tank for the fluid, a draft tube for dividing the tank into atleast two chambers, the draft tube having an open upper end and an openlower end for allowing the fluid to flow between the chambers at a lowerregion and an upper region of the tank, an axial flow pump having animpeller located in the draft tube for circulating the fluid downwardsin the draft tube and then upwards in the other chamber, a means forcreating a region of reduced pressure in a portion of the fluidcomprising a tubular member which has a region of restrictedcross-section for imparting a venturi effect to the fluid passingthrough the tubular member whereby the velocity of the fluid increasesand the pressure of the fluid decreases in the region of restrictedcross-section, a means for introducing the gas into the fluid in theregion of reduced pressure to aerate the fluid, and a means forintroducing the aerated fluid into the circulating fluid in the tank.